[Gtk] Fix FocusInEvent keeping Popover in memory after it is disposed.
[Gtk] Fix FocusInEvent keeping Popover in memory after it is disposed.

Showing a Popover window and then disposing it when it is closed would
leave the Popover window in memory if its TransientFor window was not
closed. This was because the event handler added to the TransientFor
window's FocusInEvent was not removed when the Popover was disposed.

mono/xwt,hamekoz/xwt,akrisiun/xwt,cra0zy/xwt,lytico/xwt,hwthomas/xwt,residuum/xwt,antmicro/xwt,TheBrainTech/xwt